The present disclosure relates to printing from digital images and particularly color printing. Color printing is typically accomplished by deposition of multiple colorants as for example, by printers utilizing cyan, magenta yellow and black colorants, where each colorant is represented in a channel of the digital image.
Vector halftoning is well suited for ink jet marking because the tight color-to-color registration capabilities can be used by the halftoning method to produce finer textures and a better gamut by utilizing correlation between the halftoned color planes. The method preferentially creates dot-off-dot halftone planes, which leads to those known image quality benefits. Vector halftoning requires knowledge of the values within multiple color separations to decide on marking decision in each separation. This is not an issue for many ink jet printers because jets for the various colorants are in close proximity so image data being printed for any one channel is close to image data being printed for another channel. The proximity allows for relatively low cost memory buffers.
Vector halftoning in an image path that has imaging stations for the different colorant planes at a significant separation, however, is expensive due to the need to provide large page buffers to align the data for the color planes because the different colorant values (CMYK . . . ) for a given pixel can be printed at significantly different times. In terms of memory capacity and system architecture, such processing is prohibitively expensive and otherwise impractical.
Vector halftoning could be performed in a Digital Front End (DFE) that provides the digital image to the printer. Typically, a DFE rasterizes the image from some page description language form and holds all color planes in memory simultaneously. Halftoning in the DFE avoids the cost associated with large amounts of high speed memory in the printer, which would be needed if the imaging stations are at a significant distance. However, the approach of halftoning in the DFE results in a serious limitation in the ability to maintain the calibration of a print engine. To maintain the calibration of a printer, it is desirable to apply adjustments to the digital values in the color channels to track the physical marking characteristics of a print engine as it varies in time. Continuous tone pixel values, such as 8 bit (0 to 255) values can be adjusted to maintain the calibration of the overall printing process. After halftoning is performed, pixel values within the channels are typically binary (0,1), and minor adjustment of a pixel level is very difficult to perform.
Co-pending patent application Ser. No. 12/258,871, filed Oct. 27, 2008, entitled “Image Path Utilizing Sub-Sampled Cross-Channel Image Values,” by Crean et al., describes an image path architecture for performing cost-effective cross-channel image processing. For generating the output value of a given colorant (e.g., cyan), the method utilizes pixel data for that colorant at its nominal (full) resolution, while pixel data for other colorants is utilized at a lowered resolution. For instance, color-corrected pixel values for cyan can be obtained by inputting cyan pixel values at nominal resolution into a multi-dimensional LUT, and the values for the other color dimensions are input using sub-sampled values, such as JPEG DC coefficients or DRI pixel values (Display Resolution Image). Using the nominal (full) resolutions for cross-channel processing would require a 300% increase in bandwidth for each output color, while the aforementioned disclosure utilizes JPEG DC coefficients requiring only a 4% increase. Similar gains are obtained in image buffering.
The disclosed embodiments, which relate to vector halftoning in an image path utilizing sub-sampled cross-channel image values, have the potential to provide even greater savings in image paths utilizing more than 4 colorants.